Whittaker suffered the injury Monday in a noncontact drill during the second phase of offseason workouts, the release said.

Whittaker has been with the Panthers since the 2014 season. He’s played in 53 career games, posting 536 rushing yards while averaging a respectable 4.4 yards per carry.

The injury sheds light on why the team has been so aggressive in free agency this week. On Monday, the Panthers signed former Broncos running back C.J. Anderson, and Tuesday they claimed second-year back Elijah Hood, who starred at North Carolina, off waivers.

Carolina’s backfield will be rounded out by Christian McCaffrey, rookie Reggie Bonnafon and Cameron Artis-Payne.